Recently, since pixels are becoming smaller and smaller, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-128392 describes that, a back side illumination (BSI) solid-state imaging device is proposed mainly for the purpose of increasing aperture ratio.
In a solid-state imaging device of the back side illumination type, electrons generated on the light irradiation face side are not counted as signals unless they reach a photodiode (PD) on an interconnect side. Consequently, the sensitivity is determined according to the thickness of a silicon (Si) substrate for performing photoelectric conversion (the thicker the silicon substrate, the higher the sensitivity). In this case, the number of carriers (electrons) generated by the photoelectric conversion is greater on the light irradiation face side in the silicon substrate and decreases toward the PD. In the silicon substrate near the light irradiation face side, since it is far from the PD, the electric field generated by the PD (the electric field which makes electrons concentrated in the PD) is weak. That is, the region is a weak electric field region.
Generally, an element isolation layer is formed by a p-type semiconductor layer obtained by implanting boron (B) ions to the silicon substrate. However, when boron is implanted to a deep part in the silicon substrate at high acceleration, an ion damage occurs, boron ions are scattered, and the boron concentration in the element isolation layer becomes effectively low, so that the element isolation capability deteriorates.
In the solid-state imaging device of the back side illumination type, many carriers occur on the light irradiation face side. However, they are generated in the weak electric field region. A considerable amount of carriers are moved by diffusion.
The carriers moved by the diffusion leak to adjacent pixels and color crosstalk are caused. In the BSI, there is a tendency that color crosstalk at the time of emitting short-wavelength light (blue light) occurs more than a solid-state imaging device of the surface irradiation type.
Consequently, an object of the BSI is to reduce color crosstalk of short-wavelength light. Generally, as means, the thickness of the silicon substrate in which a PD is formed is reduced, and the distance between the light irradiation face side in the silicon substrate and the PD is shortened.
However, there is a problem that reduction of the thickness of the silicon substrate causes decrease in sensitivity to green (G) and red (R) light. As described above, there is a tendency that the conventional solid-state imaging device of the back side illumination type and its manufacturing method are disadvantageous with respect to color crosstalk or the like caused by carriers generated on the light irradiation face side.